Method and device for the electrolytic coating of a metal strip

ABSTRACT

Method for the electrolytic coating of a metal strip, in which the strip forms a cathode and is moved in its longitudinal direction relative to an anode, an electrolyte flowing at least between the strip and the anode, characterized in that the flow of the electrolyte is influenced by holding a body between the strip and the anode.

[0001] The invention relates firstly to a method for electrolyticallycoating of a metal strip, in which the strip forms a cathode and ismoved in its longitudinal direction relative to an anode, an electrolyteflowing at least between the strip and the anode.

[0002] A method of this type is generally known. In the known method,the distance between the metal strip and the anode is usually held atbetween 5 and 10 cm, while the strip which is to be coated, in thetransverse direction in the vicinity of the anode, usually extends overa multiple (usually approximately 1 m) of this distance, with the resultthat a relatively narrow clearance is formed between the metal strip andthe anode. A potential difference is applied between the anode and thecathode, leading to an electric current flowing through the electrolyte.In a method in which soluble anodes are used, the electric current leadsto the dissolution of material, usually one or more metallic elements,from an anode, on the one hand, and the precipitation of the saidmaterial in a layer on the strip, on the other hand.

[0003] It is usually aimed to apply the layer at the highest possiblespeed. The rate at which the layer grows is dependent, inter alia, onthe electric current density and on the velocity at which the strip ismoved through the electrolyte. However, the electric current densityaffects not only the growth rate of the layer but also its morphology.Since undesirable dendrites are formed above a set threshold, themaximum current density is in practice limited.

[0004] The velocity of the strip is also limited in practice. If thestrip velocity were too high, given a specific, more or less limitedgrowth rate, the coating line would become too long for a specificdesired layer thickness to be reached.

[0005] It is known to use special jets on either side of the strip tospray the electrolyte into the clearance between the strip and the anodesubstantially in the transverse direction with respect to the directionof movement of the strip. In this way, the flow velocity of theelectrolyte through the clearance is increased.

[0006] One drawback of the known method is that the flow of theelectrolyte in the clearance is not sufficiently uniform, with theresult that the morphology and thickness of the deposited layer are notsufficiently uniform. Yet another drawback is that the known jets arecomplicated and are expensive to maintain and operate.

[0007] It is an object of the invention to provide a method with whichthe abovementioned drawbacks are eliminated or reduced. Another objectof the invention is to be able to increase the running velocity of thestrip, while the thickness of the layer which is deposited per unitlength in a coating line can remain at least equal. Yet another objectis to increase the efficiency of electrolysis on the strip. Yet anotherobject is to provide a less expensive method for the electrolyticcoating of a metal strip. Yet another object is to provide a method forelectrolysis in which less waste material is produced.

[0008] One or more of these objects is achieved with a method of thetype described in the first paragraph of this description in which theflow of the electrolyte is influenced by holding a body between thestrip and the anode. In this way, the diffusion boundary layer in theelectrolyte in the vicinity of the moving strip is influenced, with theresult that the precipitation of anode material on the strip can proceedmore efficiently and/or more homogeneously. A reduction in the thicknessof the boundary layer, in particular, leads to an increased rate ofdeposition of material, so that the velocity at which the strip is movedthrough the coating line can be increased.

[0009] By holding the body in the clearance between the strip and theanode, it is possible to influence the flow of the electrolyte moreuniformly than has previously been the case. According to the invention,it has been found that holding a body which is not excessively shieldingin the clearance has little if any adverse effect on the requiredpotential difference and on the uniformity of electric currentdistribution in the electrolyte in the vicinity of the strip.

[0010] The use of the invention provides additional advantages forcertain processes in which, for example, a cyanide-containingelectrolyte is used. In a process of this type, the anode efficiency isusually 100%. Since the cathode efficiency is usually lower than 100%, afraction of the exposed anode surface which corresponds to the cathodeefficiency usually consists of a non-soluble (inert) metal, in order tokeep the quantity of anode material in the electrolyte constant.However, the electrolyte breaks down at this non-soluble fraction of theanode, forming waste material. For example, a carbonate is formed fromthe cyanide, and this carbonate has to be constantly removed from theelectrolyte and disposed of as chemical waste. On the one hand, thisentails removal costs, and on the other hand raw material costs are alsoinvolved. The invention allows the efficiency at the cathode to beincreased, and consequently the drawbacks associated with the inertfraction are reduced proportionally.

[0011] Preferably, at least that section of the body which is heldbetween the strip and the anode is electrically insulating. Thisprevents the electrolysis process from being disrupted byelectrochemical activity of the body which is held between the anode andthe cathode.

[0012] Preferably, the flow of the electrolyte is influenced in such amanner that, at a certain distance from the strip, the mean velocity ofthe electrolyte, in the longitudinal direction of the strip, withrespect to the strip is higher than the velocity of the strip withrespect to the anode. This is achieved by influencing the flow in such amanner that the direction of flow of the electrolyte is as far aspossible opposite to the direction of movement of the strip. Since therelative velocity of the strip passing through the electrolyte ishigher, the boundary layer is thinner, and the precipitation of materialproceeds more successfully and more quickly.

[0013] Preferably, the body is moved. In this way, it is possible toinfluence the flow of the electrolyte more effectively without, in theprocess, requiring jets on either side of the strip. It is possible toinfluence both laminar and turbulent flows and also to convert a laminarflow into a turbulent flow. In all cases, the diffusion boundary layercan become thinner, which improves the mass transfer.

[0014] One embodiment of the method according to the invention ischaracterized in that the body, for example a perforated strip, is movedsubstantially parallel to the strip, in the opposite direction. Theoppositely directed movement of the body leads to a flow which isdirected oppositely to the direction of movement of the strip being atleast partially imposed in the electrolyte. One advantage of thisembodiment is that the distribution of the electric current densitythrough the electrolyte is not stationary, so that, on the one hand, a(usually stationary) anode is dissolved more homogeneously and, on theother hand, the layer is deposited more homogeneously on the metalstrip.

[0015] Another embodiment of the method according to the invention ischaracterized in that the body is moved in rotation about an axis, whichaxis runs substantially parallel to the strip and substantiallyperpendicular to the longitudinal direction of the strip. Given thecorrect direction of rotation, it is ensured that the electrolyte ispumped around substantially in the opposite direction to the directionof movement of the strip, with the result that the said relative stripvelocity is increased.

[0016] In this embodiment, the body is preferably rotated about itslongitudinal axis. This ensures that the electrolyte is pumped aroundsubstantially in the opposite direction to the direction of movement ofthe strip, while the conditions under which the electrolysis is carriedout fluctuate as little as possible.

[0017] The invention is also embodied by a device for the electrolyticcoating of a metal strip, comprising a housing for holding anelectrolyte, an anode, means for using the strip as a cathode, and meansfor advancing the strip in its longitudinal direction, via a path, at aspecific distance relative to the anode.

[0018] According to this aspect of the invention, the device ischaracterized in that the device furthermore comprises a body which isto be held, at least over a section thereof, in the electrolyte betweenthe anode and the path. During operation, the body influences the flowof the electrolyte, with the result that the mass transfer is improvedand material can be deposited more quickly on the strip. It has beenfound that a body which is not excessively shielding in the clearancehas little if any adverse effect on the potential difference between theanode and the strip required during operation and on the uniformity ofthe electric current distribution of the electrolyte on the strip.

[0019] Preferably, at least that section of the body which is to be heldbetween the anode and the path is electrically insulating. This preventsthe bodies which are to be held between the anode and the path frombeing electrochemically active.

[0020] The path in which the metal strip is to be moved past the anodecomprises an active area, where the strip is coated during operation,and also comprises an open area, which open area is free of an imaginaryshadow formed by a perpendicular projection of a body which, duringoperation, at least over a section thereof, is situated between theanode and the path. Preferably, the open surface comprises more than 60%of the active area of the path. It has been found that under thiscondition the body does not shield the anode excessively from the path,with the result that the current density distribution and the requiredpotential difference in the customary electrolysis processes are notadversely affected or are only slightly adversely affected as a resultof the body, if this condition is complied with.

[0021] Preferably, the body extends parallel to the path. This ensuresthat the flow of the electrolyte, during operation, is influenced ashomogeneously as possible along the path.

[0022] The device preferably comprises means for moving the body. Inthis way, it is possible to influence the flow of the electrolyte moreeffectively, without requiring jets on either side of the strip.

[0023] In one embodiment of the device according to the invention, thebody comprises a perforated strip. In this way, the flow of theelectrolyte is influenced homogeneously over the entire active area ofthe path. The perforation serves to create a passage for the material ofthe anode and the electric current. When the strip is moved in theopposite direction to the direction of movement of the metal strip whichis to be coated, the electrolyte will also be moved with the strip, andthe velocity of the strip with respect to the electrolyte will beincreased as a result. A further advantage of a perforated strip is thatthe distribution of the electric current density does not remainstationary while the device is operating, with the result that the anodeis dissolved more uniformly.

[0024] In another embodiment, the device comprises two or more bodieswhich are to be held at least in the electrolyte between the anode andthe path. This once again results in homogeneous influencing of the flowof the electrolyte. If desired, the bodies can rotate about an axiswhich is parallel to the path and is oriented in the transversedirection of the direction of movement of the strip in the path. Thisembodiment is relatively easy to incorporate in an existing device.

[0025] Preferably, the distance from the bodies to the path is identicalfor each of the bodies. The result is a more uniform coating.

[0026] The invention will now be explained with reference to anexemplary embodiment of the method and device according to theinvention, with reference to the drawing, in which:

[0027]FIG. 1 shows a diagrammatic cross section through an exemplaryembodiment of the device according to the invention;

[0028]FIG. 2 shows an enlarged excerpt from FIG. 1;

[0029]FIG. 3 shows, for various rotational frequencies of the body in asimulation unit such as that shown in FIG. 2, the flow velocity of theelectrolyte as a function of the distance from the axis of rotation ofthe body;

[0030]FIG. 4 shows the experimentally determined cathode efficiency on arotating cylindrical cathode during electrolytic coating with copper ina cyanide bath;

[0031]FIG. 5 shows the flow velocity of the electrolyte at differentlocations in the cell, in the simulation unit shown in FIG. 2;

[0032]FIG. 6 shows the flow velocity of the electrolyte past the stripat a line which lies 0.5 cm away from the strip, in the simulation unitshown in FIG. 2;

[0033]FIG. 7 shows the flow velocity of the electrolyte as a function ofthe distance from the axis of rotation of the body with a stationary andmoving strip and with a stationary and rotating body, in the simulationunit shown in FIG. 2;

[0034]FIG. 8 diagrammatically depicts, in cross section, the geometry ofa simulation unit which is used to calculate the electrical propertiesof the device;

[0035]FIG. 9 shows the relative distribution of the electric currentdensity through the electrolyte in the vicinity of the surface of thecathode, for various dimensions of the body;

[0036]FIG. 10 shows the relative distribution of the electric currentdensity through the electrolyte in the vicinity of the surface of thecathode, for various dimensions of the cell; and

[0037]FIG. 11 shows the relative distribution of the electric currentdensity through the electrolyte in the vicinity of the surface of thecathode in the embodiment of the invention as illustrated in FIG. 2, inwhich the body comprises a rotating cylindrical body.

[0038]FIG. 1 shows a device for coating a metal strip with the aid ofelectrolysis, including a housing 6, a metal strip 1, an anode 4 andmeans for advancing the strip in its longitudinal direction, in thedirection of the arrow, via a path at a certain distance from the anode,for example a conveyor roller 2. The housing 6 is filled with anelectrolyte 3. Metal strip 1 is used as cathode. A potential differenceis applied between metal strip 1 and anode 4, with the result that anelectric current passes between the anode and the cathode, andelectrolysis can take place. During electrolysis, material is depositedon the metal strip, so that it is coated with a layer.

[0039] According to the invention, the device also comprises a body 5 atleast partially between the anode and the path of the metal strip. Inthe embodiment as shown in FIG. 1, there are a number of rod-like bodies5 at equal distances from the metal strip. The rod-like bodies 5 canrotate in the direction of the arrows. Rotation of the bodies causes theflow of electrolyte to be influenced. In this way, the boundary layerwhich is situated in the electrolyte in the vicinity of the moving stripis influenced in such a manner that the deposition of material on thestrip proceeds more successfully.

[0040] Usually, the mass transfer of deposition on a long flat strip, ata specific current density, is virtually proportional (the logarithm ofproportionality is approximately 0.9) to the velocity at which the stripis moved through the electrolyte. By influencing the flow of electrolytein such a manner that the relative velocity of the strip with respect tothe electrolyte increases, the mass transfer at the metal strip canincrease.

[0041] In FIG. 1, the box A indicates the section of the device which isillustrated on an enlarged scale in FIG. 2. The reference numbering usedin FIG. 2 corresponds to the reference numbering used in FIG. 1. On thebasis of the geometry shown in FIG. 2, a study of the distribution ofthe flow velocities of the electrolyte as a result of a regular row ofcylinders which are positioned parallel to the metal strip and rotateabout their longitudinal axis was carried out. For this study, theparameters selected were a cell width B of 10 cm, a cell height H of 10cm, in the centre of which a cylindrical body 2, which has a radiusR=1.5 cm, rotates at a specific frequency. The study was carried outwith the aid of numeric CFX calculations, using periodic boundaryconditions so that the effect of adjacent bodies is also included in thestudy.

[0042]FIG. 3 shows the flow velocity ν of the electrolyte in metres persecond as a function of the distance r on line X-X from the axis ofrotation of the body 2, with the strip 1 being stationary. Line 10 showsthe flow velocity as a result of the body being rotated about itslongitudinal axis at a rotational frequency of 10 Hz. At this rotationalfrequency, the velocity of the cylinder surface is 0.94 m/s. It will beclear that when the body rotates the electrolyte is set in motion.Within a few millimetres of the cylinder surface, the velocity of theelectrolyte has halved. There then follows a range in which the flowvelocity decreases at approximately 1/r+1/(B−r), which corresponds to apotential approximation of two bodies with mirror symmetry in the planeof the strip. Finally, a thin boundary layer is formed close to thestrip 1, in which boundary layer the flow velocity of the electrolyte isadapted to the velocity of the strip (which in this case is stationary).The formation of this boundary layer is beneficial to the mass transfer.

[0043] The fact that this also improves the cathode efficiency isillustrated on the basis of an experiment in which the efficiency at acylindrical cathode with a diameter of 1.2 cm was systematicallydetermined by making this cathode rotate at different rotationalfrequencies Ω of between 1 and 26.8 Hz in a cyanide bath with acomposition of 112.8 g/l of CuCN (80 g/l of Cu)+135.4 g/l of NaCN+80 g/lof Na₂CO₃, during copper electrolysis with a current density of 500Am⁻². The cathode efficiency is determined by anodically (at an anodeefficiency of 100%) re-dissolving the copper which has precipitated onthe cathode surface within a set time, a noticeable change in thevoltage drop indicating the moment at which all the copper hasdisappeared from the surface. It is known that the mass transfer with arotating cathode of this nature is proportional to a 0.7 power of thefrequency. Therefore, in FIG. 4 the cathode efficiency, CE, is plottedagainst Ω^(0.07). It can be seen from FIG. 4 that at a bath temperatureof 70° C. the cathode efficiency at the cylinder at 1 Hz rotation isapproximately 75%, and increases proportionally to Ω^(0.7) up to amaximum of approximately 93%. The efficiency does not increase furtherif the rotational frequency is increased further than approximatelyΩ^(0.7)≈5 per Hz.

[0044]FIG. 4 shows that improvement in the mass transfer (reduction inthe size of the boundary layer) increases the cathode efficiencynoticeably. Assuming that the mass transfer, in the case of a flatcathode, improves directly proportionally to the velocity of the strippassing through the electrolyte, an increase in the relative velocity ofthe strip by a factor of 5 is sufficient to raise the cathode efficiencyfrom 75% to 93%.

[0045]FIG. 3 also shows the line 11 which represents the velocityprofile which was found for a rotational frequency of 20 Hz, and theline 12 shows the velocity profile for the rotational frequency of 40Hz. The mean flow velocities of the electrolyte which are derived fromFIG. 3 and are caused by the rotating bodies are shown in the followingtable: Corresponding line Rotational frequency of Mean flow velocity onnumber in FIG. 3 the body (Hz) line X-X (m/s) 10 10 0.35 11 20 0.60 1240 1.37

[0046] At a flow velocity of 0.34 m/s, the required factor of 5 in masstransfer in order to provide the maximum improvement in cathodeefficiency with a constant strip velocity, is achieved in this exampleat 40 Hz.

[0047] The study has also shown that the mean flow velocity of theelectrolyte increases by approximately the third power with the radiusof a cylindrical body. If desired, this fact is also used in the designof a device for electrolysis.

[0048] In FIG. 5, line 12 once again shows the profile, on line X-X, ofthe flow velocity ν of the electrolyte as a result of a body rotating at40 Hz. Line 13 in FIG. 5 represents the local velocity of theelectrolyte on line Z-Z. Over the entire width of the cell, the velocityon line Z-Z is lower than the velocity on line X-X. FIG. 6 shows thevelocity as a function of the position y on an axis Y-Y which runsparallel to the metal strip at a distance of 4.5 cm from the axis ofrotation (0.5 cm distance from the metal strip). The value y=5.0 cmcorresponds to the intersection of line Y-Y and line X-X. It can be seenfrom the figure that the expected mass transfer behind the rotatingbodies is higher by approximately a factor of 2 than the mass transferin the centre between two adjacent rotating bodies.

[0049]FIG. 7 shows a study which is comparable to that shown in FIG. 3,where line represents the flow velocity ν of the electrolyte on line X-Xwith a stationary strip and a cylindrical body rotating at 10 Hz. Line14 represents the velocity distribution on line X-X for the situation inwhich the body is not rotating and the strip is moved at 1.0 m/s in itslongitudinal direction through the device. Apart from the boundary layerwhich is formed in the vicinity of the stationary body, this combinationwould correspond to the situation in which there is no body 5, as in theprior art. Finally, line 15 shows the effect of rotating the body at 10Hz with a moving strip. It is clear that the boundary layer becomesthinner and the velocity gradient in the vicinity of the strip is higherwhen the body rotates. It will be understood that the velocity gradientincreases still further at a high rotational frequency.

[0050] Since the cathode efficiency is increased, it is also possible toincrease the velocity at which the strip is advanced. As a result, it ispossible, using the same device and the same current density, to coatmore metres of strip per unit time to the same layer thickness.

[0051] It can be seen from the above that the embodiment with rotatingcylindrical bodies has a positive effect on the formation of a boundarylayer in the vicinity of the surface of the metal strip which is to becoated. Naturally, it is possible to use variations, such as forexamples bodies which are provided with blades, brushes or are formed insome other way in order to improve the transfer of motion to theelectrolyte.

[0052] The text which follows will describe how the positioning ofbodies influences the distribution of the electric current densitythrough the electrolyte, and how the influence on homogeneity ofprecipitation of material on the strip can be minimized.

[0053] It is known that, with regard to an electric current density,above a certain threshold the morphology of the deposited layer isdominated by dendrites, resulting in a layer which has undesiredproperties. A maximum current density of between approximately 60 and80% of this threshold is generally used, which in practice represents acurrent density of approximately 500 Am⁻². To be able to use a meancurrent density which is as high as possible, it is important for thedistribution of the current density in the vicinity of the surface ofthe metal strip to be as even as possible.

[0054] The distribution of the current density also has to be kept aseven as possible in particular when coating a metal strip with an alloy(such as for example Cu—Zn), since the composition of the alloy which isdeposited is dependent on the current density. If the current densityvaries excessively, the composition of the layer is not sufficientlyhomogeneous. It is usually attempted to keep the current density of theelectrolyte on the strip (i) relative to the mean current density(i_(avg)) within a range of 0.9<i/i_(avg)<1.1.

[0055] Furthermore, the potential difference required should be kept aslow as possible, in order to minimize dissipation. The voltage dropacross the electrolyte which is deemed to be the maximum acceptable for,for example, the electrolytic coating of steel with copper is 7.0 V,while the desired value is between 5.0 and 5.5 V.

[0056] The distribution of the electric current density at a location yon the path and the required potential difference can be calculatedaccurately. FIG. 8 shows, in cross section, the geometry of a simulationcell at which calculations of the electric current density were carriedout using the method known as the boundary elements method. Thecalculations are based on Laplace's equation and Ohm's law. Thecalculations assume a series of rod-shaped bodies. The metal strip(cathode) is imagined to be on one of the vertical sides, with the anodeon the opposite vertical side. From a repeating series of this type, asimulation cell was taken, as shown in FIG. 8. It is assumed that thecell is filled with a medium for which the conductivity is equal to κ=10Ω⁻¹m⁻¹, which corresponds to the electrolytes which are customarilyemployed for the electrolytic coating of steel. Furthermore, a cellwidth of B=10 cm, a cell height of HH=10 cm, and a body of l=2.0 cm widewere selected. The half height, hh, of the body was varied in thecalculations.

[0057]FIG. 9 shows the distribution of the electric current density inthe vicinity of the surface of the metal strip for various values of thehalf height hh of the body, varying from 1.0 to 9.0 cm inclusive, as afunction of the position y on the strip in the simulation cell shown inFIG. 8. The various types of lines correspond to the legend, in whichthe associated values for hh (in cm) and the voltage drop across theelectrolyte (in V) are given. The distribution of the current density isshown as the relative current density i(y)/i_(avg) compared to the meancurrent density i_(avg). It can be seen that the distribution of thecurrent density becomes more even as the height of the body becomessmaller. If i_(avg) is set at 70% of the threshold, the maximum currentdensity, in the event of a deviation by a factor i/i_(avg)<1.4, stillremains below the threshold. As shown in FIG. 9, this is the case forbodies for which the half height hh of the body is less than or equal to4.0 cm. With bodies with a half height of 1.0 cm or less, therequirement of 0.9<i/i_(avg)<1.1 is satisfied.

[0058] The voltage drop across the electrolyte associated with a meancurrent density of i_(avg)=500 Am⁻² is also indicated in the legend toFIG. 9. With the selected κ of 10 Ω⁻¹m⁻¹ in a 10 cm wide cell which isfilled only with electrolyte, the voltage drop at i_(avg)=500 Am⁻² is5.0 V. As can be seen from the figure, the presence of a body causes anincrease in the voltage drop across the electrolyte. The higher the hhof the body, the higher the voltage drop. A body with a half height ofhh=4.0 cm causes a voltage drop of 7.0 V and is therefore stillacceptable.

[0059] Both the distribution of the current density and the voltage dropacross the electrolyte can be improved further by positioning a greaternumber of smaller bodies between the anode and the strip. FIG. 10 shows,for a number of simulation cells with a width of B=10 cm and with a bodyof l=2.0 cm wide, the current density distribution as a function of theposition on the strip with respect to the height of the cell (y/HH), andthe voltage drop, with the relative height of the body with respect tothe height of the cell (hh/HH) being kept constant. The legend shows,for every curve, the associated HH, hh (both in cm), and the voltagedrop across the electrolyte (in V). It can be seen that with anincreasing number of smaller bodies the current density becomes moreeven and at the same time the voltage drop across the electrolytebecomes lower.

[0060] It is clear from FIG. 9 and FIG. 10 that under certain conditionsthe presence of a body between the anode and the strip does not have tounacceptably disrupt the current density and the potential differencerequired. In certain cases, the disruption is even negligible.

[0061]FIG. 11 shows the distribution of the electric current density fora 10 cm wide cell, in which the same cylindrical body (with a radius of1.5 cm) as that shown in FIG. 2 is held, for different cell heights HHranging from 2.0 to 5.0 cm, as indicated in the legend. The situation inwhich HH=5.0 cm corresponds to the calculations from FIGS. 3, 5, 6 and7. In this situation, the requirement of 0.9<i/i_(avg)<1.1 is satisfied,and moreover the voltage drop across the electrolyte (at 500 Am⁻²), ascan be read off from the legend, is below 6.0 V. The variation in HHcorresponds to reducing the distances between adjacent bodies. Thedistribution of the current density becomes more even as HH becomessmaller, to the detriment of the voltage drop.

[0062] It is possible for the diffusion boundary layer and the localvariation in the current density to be adapted to one another. Thistakes place as follows. It can be seen from FIG. 6 that, for a certaingeometry, the flow velocity of the electrolyte (and therefore also theexpected mass transfer) behind the rotating bodies is higher byapproximately a factor of 2 than in the centre between two adjacentrotating bodies. The velocity distribution over the strip can be mademore even by reducing the distance between adjacent bodies. It can beseen from the study of the electric current density that the electriccurrent density through the electrolyte just behind the bodies is lowerthan between the bodies. Consequently, with a uniform boundary layer,the growth rate of the layer behind the rotating bodies would in fact belower. As has emerged from the study, the distribution of the electriccurrent density can be varied independently of the distribution of theboundary layer. Since the two distributions have an opposite effect onthe mass transfer in the vicinity of the surface of the strip, it ispossible to design an optimum geometry in which the mass transfer acrossthe strip becomes as homogeneous as possible.

[0063] The invention has been explained above on the basis of elongaterotating bodies, but the invention is not limited thereto. By way ofexample, an embodiment in which a perforated strip is moved, in theopposite direction to the movement of the strip to be coated, past thestrip has already been offered in this application.

1. Method of electrolytically coating of a metal strip, in which thestrip forms a cathode and is moved in its longitudinal directionrelative to an anode, whereby an electrolyte is flowing at least betweenthe strip and the anode, and a body is held between the strip and theanode for influencing the flow of the electrolyte, characterized in thatthe body is moved.
 2. Method according to claim 1, characterized in thatat least that section of the body which is held between the strip andthe anode is electrically insulating.
 3. Method according to claim 1 or2, characterized in that the flow is influenced in such a manner that,at a certain distance from the strip, the mean velocity of theelectrolyte, in the longitudinal direction of the strip, with respect tothe strip is higher than the velocity of the strip with respect to theanode.
 4. Method according to any one of claims 1 to 3, characterized inthat the body is moved substantially parallel to the strip, in theopposite direction.
 5. Method according to any one of claims 1 to 3,characterized in that the body is moved in rotation about an axis, whichaxis runs substantially parallel to the strip and substantiallyperpendicular to the longitudinal direction of the strip.
 6. Methodaccording to claim 57 characterized in that the body is rotated aboutits longitudinal axis.
 7. Device for electrolytically coating of a metalstrip, comprising a housing for holding an electrolyte, an anode, meansfor using the strip as a cathode, and means for advancing the strip inits longitudinal direction, via a path, at a specific distance relativeto the anode, and a body which is to be held, at least over a sectionthereof, in the electrolyte between the anode and the path,characterized in that the device furthermore comprises means for movingthe body.
 8. Device according to claim 7, characterized in that at leastthat section of the body which is to be held between the anode and thepath is electrically insulating.
 9. Device according to claim 7 or 8,characterized in that thee path comprises an active area, where thestrip is coated during operation, and an open area, which open area,during operation, is free of an imaginary shadow formed by aperpendicular projection of a body which is situated between the anodeand the path, the open surface comprising more than 60% of the activearea.
 10. Device according to one of claims 7 to 9, characterized inthat the body extends substantially parallel to the path.
 11. Deviceaccording to claim 10, characterized in that the body comprises aperforated strip.
 12. Device according to one of claims 7 to 10,characterized in that two or more bodies, at least over a section ofeach body, are situated between the anode, and the path.
 13. De,iceaccording, to claim 12, characterized in that the distance from thebodies to the path is identical for each of the bodies.